GB2163669A

GB2163669A - Apparatus for the separation of a gaseous mixture by pressure swing adsorption (PSA)

Info

Publication number: GB2163669A
Application number: GB08515381A
Authority: GB
Inventors: Michael Ernest Garrett
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1984-06-21
Filing date: 1985-06-18
Publication date: 1986-03-05
Also published as: GB2163669B; JPH0669525B2; ZA854528B; GB8515381D0; GB8415929D0; JPS6178417A

Abstract

In an apparatus for the separation of a gaseous mixture using PSA techniques, two vessels 8, 10 each containing a bed of adsorbent material 12, 14 are interconnected at their discharge ends by a pipeline 80 which incorporates a flow control valve 82. The pipeline 80 at least at one end extends into the interior of a vessel 8, 10 for at least 25% of the length of the vessel. <IMAGE>

Description

SPECIFICATION Improved method and apparatus for the separation of a gaseous mixture The present invention relates to method and apparatus for the separation of a gaseous mixture using pressure swing adsorption techniques (PSA).

It is known from United Kingdom published patent application No. 2073043A to separate nitrogen from air using an adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its more rapid adsorption of oxygen than of nitrogen. The adsorbent is usually molecular sieve carbon.In operation, a bed of the adsorbent is put through a cycle which includes an adsorption step during which time air is pumped through the bed, most of the oxygen and a proportion of the nitrogen and substantially all of the carbon dioxide and water vapour in the feed are adsorbed and a nitrogen-rich product gas is supplied from the outlet of the bed; and a desorption or regeneration step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step.

In practice, two adsorbent beds are employed and operated on similar cycles which are sequenced to be out of phase with one another by 1800 so that when one bed is on its adsorption step, the other bed is on its regeneration step and vice-versa. Between the adsorption and the regeneration steps, the pressures in the two beds are equalised by connecting the two bed inlets together and connecting the two bed outlets together. With these connections made, the gas within the void spaces of the bed which has just completed its adsorption step flows into the bed which has just completed its regeneration step by virtue of the pressure difference which exists between the beds at this stage. This equalisation step is found to be beneficial in maximising the product output because the gas in such void spaces will have already become somewhat enriched in nitrogen.

The apparatus and process described in United Kingdom published patent application No.

2073043A is efficient and offers a high percentage purity of nitrogen as product gas.

It is an aim of the present invention, by improving the apparatus described in United Kingdom published patent application No.

2073043A to increase the efficiency and the product purity when operating a pressure swing adsorption process with said apparatus.

According to the present invention, an apparatus for the separation of a gaseous mixture by pressure swing adsorption comprises first and second vessels, each containing a bed of adsorbent material which adsorbs preferentially at least one constituent of a feed gas mixture, each vessel at one end having an inlet for the feed gas mixture and at an opposite end an outlet for product gas; a first pipeline incorporating a valve for permitting communication between the vessels at their inlet ends when said valve is open, and a second pipeline incorporating a valve for permitting communication between the vessels at their outlet ends when said valve is open, in which at least one end of the second conduit extends into the interior of the first vessel for at least 25% of the length of the first vessel for at least the second conduit at each end extends into the interior of an individual vessel for at least 25% of the length of the vessel otherwise ends on the first vessel at a position at least quarter of the vessel's length away from its product outlet end.

The invention also provides a method of separating a gaseous mixture by pressure swing adsorption employing a plurality of adsorbent beds, including the step of causing gas to flow from the bottom region of a first bed that has finished an adsorption step to the bottom region of a second bed that has finished a regeneration step and to flow from an upper region of said first bed into an upper region of said bed at a level in said second bed at least one quarter of the way down the bed.

Preferably, the second conduit at each end extends into the interior of an individual vessel for at least 25% of the length of the vessel or otherwise ends in the first vessel at a position at least a quarter of the vessel's length away from its product outlet end.

Embodiments of the invention will now be described by way of example, reference being made to the Figures of the accompanying diagrammatic drawings: Figure 1 is a schematic diagram of a known apparatus for the separation of nitrogen from air; Figure 2 is a schematic diagram of an apparatus for the separation of nitrogen from air according to the present invention; and Figure 3 is a schematic diagram of a further apparatus for the separation of nitrogen from air in accordance with the present invention.

Referring first to Fig. 1, this illustrates the known apparatus as described and shown in United Kingdom published patent application No. 2073043A for the separation of nitrogen from air. The apparatus includes an air feed line 2 leading to a compressor 4. The outlet from the compressor 4 communicates with an air inlet pipeline 6. The air inlet pipeline 6 is able to be placed in communication with either one of vessels 8 and 10 containing beds 12 and 14 of carbon molecular sieve adsorbent.

Valve 16 is operable to place the bottom of bed 12 in communication with the inlet pipe 6 or to deny communication between the bed 12 and the inlet pipeline 6. Analogously, valve 18 is operable to place the bottom of the bed 4 in communication with the inlet pipeline 6 or to deny such communication.

The apparatus includes an outlet pipeline 20. A stop valve 22 is operable to place the top of the bed 12 in communication with the pipeline 20 or to prevent communication between the bed 12 and the pipeline 20. Analogously, a stop valve 24 is operable to place the top of the bed 14 in communication with the pipeline 20 or to deny such communication.

The apparatus includes an outlet pipeline 34 for waste gas. A valve 36 is operable to place the bottom of the bed 12 in communication with the pipeline 34 or may be closed to deny such communication. A valve 38 is operable to place the bottom of the bed 14 in communication with the pipeline 34 or, when closed, deny such communication. The pipeline 34 communicates with a vacuum pump 42. The outlet of the vacuum pump 42 communicates with the pipeline 34 for the discharge of waste gas from the apparatus. In communication with the pipeline 34 upstream of a valve 40 is a vent pipeline 46 communicating with the atmosphere. In the pipeline 46 is a valve 48. If desired, the vent pipeline 46 and the valve 48 may be omitted from the apparatus. Alternatively, the vacuum pump 42 and valves 40 and 48 may be omitted.If the vacuum pump is omitted, the bed will be subject to atmospheric pressure to effect regeneration of the bed which previously adsorbed oxygen. If the vent pipeline 46 and the valve 48 are omitted, the beds 12 and 14 will be subjected to a sub-atmospheric pressure in order to effect regeneration of a bed which previously adsorbed gas.

Pipeline 50 connects together the bottoms of vessels 8, 10. A similar pipeline 52 connects together the tops of vessels 8, 10. In the pipeline 50 is disposed a stop valve 54 and in the pipeline 52 is disposed a stop valve 56. When the valves 54 and 56 are open, there is communication between the beds 12 and 14.

Describinhg very briefly a process using the apparatus in Fig. 1 for the separation of nitrogen from air and assuming at the start of a cycle of operations that the bed 12 is at a sub-atmospheric pressure being subjected to vacuum created by the pump 42 and the bed 14 is at its maximum pressure being in communication with the compressor 4. The cycle starts with the beds 12, 14 being placed in communication with one another through the pipelines 50 and 52. In this phase of the cycle valves 16, 18, 22, 24, 36 and 38 are all in their closed position and valves 54 and 56 are open. Unadsorbed nitrogen-rich gas in the spaces between individual particles of adsorbent in the bed 14 flow through pipelines 50 and 52 into the bed 12. The unadsorbed gas in the bed 14 which passes to the bed 12 is rich in nitrogen.As it passes to the bed 12, so the pressure in the bed 12 increases from below atmospheric pressure and the pressure in the bed 14 undergoes a corresponding reduction.

The cycle will then continue with the bed 12 performing an adsorption step whilst the bed 14 is regenerated in a manner well known in the art.

It will be apparent from this known apparatus and process that equalisation of the two beds 12, 14 is carried out at both ends of the vessels 8, 10 with the result that the regenerated bed receives "tail-end product" immedi ately before its one adsorption step. This "tail-end gas" is of lower purity than product gas and the purity will get worse with the rapid depressurisation, regeneration and advance of the adsorption front in the exhausting bed.

In order to overcome these disadvantages, the apparatus of Fig. 1 has been modified as will be explained by reference to Fig. 2. In Fig. 2 it will be seen that the pipeline 52 and flow stop valve 56 have been removed and replaced by pipeline 80 and flow control valve 82. It will be seen that the pipeline 80 at each end extends into the interior of an individual vessel 8, 10 for at least 25% of the length of the vessel. This apparently simple modification to the apparatus avoids the disadvantages referred to with reference to Fig.

1 by arranging for the equalising gas to pass into the recently regenerated bed 12 at about 1/4 to 1/3 from the top of the vessel so that the gas filling the top of the regenerated bed 12 has passed through regenerated adsorbent material and is therefore of high purity. This has the result of improving the purity of the initial gas received from the bed 12 and therefore the yield of useful product gas.

Referring now to Fig. 3, this is a further modification of the apparatus as described and illustrated with reference to Fig. 1 in that again pipeline 52 and stop valve 56 are removed and replaced by a first pipeline 90 with a flow control valve 92 and a second pipeline 94 with a flow control valve 96. It will be seen that each pipeline 90, 94 at one end extends into the interior of a vessel 8, 10 for at least 25% of the length of the vessel.

This has exactly the same effect as described with reference to Fig. 2. However, it does require rather more pipeline and one extra flow control valve.

Although reference has been made in the above described embodiments to the production of nitrogen from air, the apparatus could be used for the separation of any one constituent of a gaseous mixture using PSA techniques.

Claims

1. An apparatus for the separation of a gaseous mixture by pressure swing adsorption comprising first and second vessels each containing a bed of adsorbent material which adsorbs preferentially at least one constituent of a feed gas mixture, each vessel at one end having an inlet for the feed gas mixture and at an opposite end an outlet for product gas; a first pipeline incorporating a valve for permitting communication between the vessles at their inlet ends when said valve is open and a second pipeline incorporating a valve for permitting communication between the vessels at their outlet ends when said valve is open, in which at least one end of the second pipeline extends into the interior of the first vessel for at least 25% of the length of the first vessel or otherwise ends in the first vessel at a position at least a quarter of the vessel's length away from its product outlet end.

2. An apparatus as claimed in claim 1, in which the second pipeline at each end extends into the interior of an individual vessel for at least 25% of the length of that vessel or otherwise ends in the first vessel at a position at least a quarter of the vessel's length away from its product outlet end.

3. An apparatus as claimed in claim 1, in which a third pipeline is provided incorporating a valve for permitting communication between the vessels at their outlet ends when said valve is open, at least one end of the third pipeline extending into the interior of the second vessel for at least 25% of the length of the second vessel or otherwise ends in the first vessel at a position at least a quarter of the vessel's length away from its product outlet end.

4. An apparatus for the separation of a gaseous mixture by pressure swing adsorption constructed and arranged substantially as hereinbefore described with reference to Fig. 2 or Fig. 3 of the accompanying drawings.

5. A method of separating a gaseous mixture by pressure swing adsorption employing a plurality of adsorbent beds, including the step of causing gas to flow from the bottom region of a first bed that has finished an adsorption step to the bottom region of a second bed that has finished a regeneration step and to flow from an upper region of said first bed into an upper region of said bed at a level in said second bed at least one quarter of the way down the bed.

6. A method as claimed in claim 5, in which gas is taken from said upper region of said first bed at a level at least one quarter of the way down the bed.

7. A method of separating a gaseous mixture, substantially as herein described with reference to Fig. 2 or Fig. 3 of the accompanying drawings.